Methylation of cytosine in the genomic DNA plays an important role in mammalian embryogenesis. DNA methyltransferase activity, which contributes mainly to the maintenance of the methylation pattern during proliferation, is under the control of the cell cycle, its activity being higher in the S phase than in the other phases (Adams, R.L.P., 1990, Biochem. J. 265, 309-320). In the present study, we examined how DNA methyltransferase is regulated in the cells arrested at S phase by aphidicolin treatment. The activity and protein levels of DNA methyltransferase in the nuclei were kept constant in proliferating mouse erythroleukemia cells, and increased about twofold after 6 h incubation in the presence of aphidicolin. This increase of DNA methyltransferase levels by aphidicolin treatment was positively correlated with the cell population at S phase. De novo synthesis of DNA methyltransferase protein was increased by the treatment. In addition, the relative half life of pulse labeled DNA methyltransferase was prolonged by aphidicolin treatment. Both increase in synthesis and prolongation of half life of DNA methyltransferase in S phase contributed to the increase of the activity and the protein levels by aphidicolin treatment. Prolongation of half life was abolished by cycloheximide, suggesting that newly synthesized protein(s) with a short half life participated in the degradation of DNA methyltransferase.